1. Field of the Invention
The present invention relates to wireless systems, and more particularly to highly integrated semiconductor devices that fully include 5-GHz transceivers suitable for untethered computer data networking.
2. Description of Related Art
John D. O'Sullivan, et al., describe portable computer wireless local area network devices that operate in excess of 10 GHz in U.S. Pat. No. 5,487,069, issued Jan. 23, 1996, (herein “O'Sullivan '069”). One object of such devices is to allow portable computer users to access the enterprise's LAN untethered and from any location in several buildings on a campus. Plug-in connectors are possible in such situations, but they are not very convenient. Unfortunately, prior art wireless systems have been limited to rather modest data rates. Such small bandwidth can be aggravating in modern Internet uses.
Carrier frequencies in the ultra-high frequency (UHF) radio bands and above can naturally carry very high modulation rates, so more data bandwidth is inherently available. But UHF and microwave radio signals are subject to multipath interference that can corrupt communications. O'Sullivan '069 summarizes some of the other problems encountered by prior art systems when trying to operate at 10-GHz and higher.
A method of converting data into symbols that are used to modulate the radio carrier is offered by O'Sullivan '069 to overcome the problems inherent in spread spectrum systems. The use of symbols establishes many parallel sub-channels that each have modulation periods much longer than any multipath delays that might confuse demodulation. Such Patent is incorporated herein by reference. In effect, O'Sullivan '069 describe the basic coded orthogonal frequency division multiplexing (COFDM) called for in the IEEE-802.11a wireless LAN Specification.
Michael Fattouche, et al., describe a method of OFDM wireless communication amongst a number of transceivers, in U.S. Pat. No. 5,282,222, issued Jan. 25, 1994, (herein “Fattouche '222”). They observe that portable uses require that the power demands of the transceivers be kept to a minimum. The use of unlicensed bands is seen as an advantage, but the operating parameters of the transceivers are constrained by law. (E.g., 47 CFR §15.407.) Fattouche '222 describe implementations in which carrier and timing recovery is not needed by carefully choosing certain parameters for differential coding. The IEEE-802.11a Specification does not use differential coding, and quite deliberately includes a preamble transmission so that carrier and timing can be recovered.
The IEEE-802.11a burst transmission begins with a two-part preamble, e.g., a short preamble and a long-preamble. The exact boundary point between the short and long preambles is important to the receiver's subsequent demodulation process, and must be found quickly in an environment where the carrier frequency and code phrase are uncertain. Signal fading, multipath interference, and channel distortion can make signal acquisition less certain in a typical receiver.
The quality of carrier frequency-offset estimation must be such that the relative error between actual and estimated values does not exceed approximately one percent of the frequency spacing between consecutive sub-carriers, e.g., 3.125 KHz at the higher data rate. About one percent is necessary at 54 Mbps maximum rate easing to about ten percent for the lowest 6 Mbps rate for negligible degradation of the data transmission. In order to reach this target precision, frequency offset estimation may be carried out in two successive stages. Coarse and fine estimates may be derived from the processing of the short preamble symbols. The residual frequency error usually remaining after coarse frequency offset correction depends on the signal levels but is about one percent, e.g. ±2 KHz for signals of sufficient strength for demodulation at the maximum 54 Mbps rate. The physical layer convergence procedure (PLCP) preamble field is used for receiver synchronization and comprises ten short symbols and two long symbols. See, IEEE 802.11a-1999, §17.3.3. In common parlance, these are called the “short preamble” and the “long preamble.”
Transmitter and receiver frequency offset errors as estimated according to the above description need to be removed in order for a receiver to track the transmitted signal and demodulate it properly. At least two conventional methods exist to remove such offsets. A first feeds back a correction signal to a VCO driving local oscillator to cancel the offsets. A second method accounts for the offsets in subsequent digital signal processing.
Radios-on-a-chip (RoC) are now being promoted by several companies, e.g., Atheros Communications (Sunnyvale, Calif.) which markets its AR5000 chipset. Such put complete 5.15–5.35 GHz transceivers on a chip, and need only few external filters, a transmit/receive switch and a crystal to operate. High power is not easily achievable on a single CMOS chip, so makers like Atheros and Radiata Communications (San Jose, Calif.) produce radio-on-a-chip devices that operate at low signal power-output levels. External transmitter power amplifiers and receiver low-noise amplifiers are then added if needed.